This application is based on Japanese Patent Applications No. 09-064357 filed on Mar. 18, 1997, No. 09-308253 filed on Nov. 11, 1997, No. 09-308254 filed on Nov. 11, 1997, and No. 09-308255 filed on Nov. 11, 1997, the contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to apparatus for and method of obtaining a two-dimensional image of an object by means of an image sensor and a scanning mechanism. Such apparatus and method are especially applied to a digital camera for photographing an object by means of a one- or two-dimensional image pickup device such as a CCD image sensor.
2. Description of the Prior Art
Portable digital cameras are now on the market as one of the simplest types of image input means for individual use. Certain of the digital cameras devised heretofore incorporate two-dimensional image pickup devices (i.e., area sensors), incorporate flash lamps to take a photograph with a blare of flashlight in the same way as cameras for silver halide films. Others of the digital cameras are called line sensor cameras because they incorporate one-dimensional image pickup devices (i.e., line sensors) combined with auxiliary scanning mechanisms such as means for rotating a mirror, whereby higher resolution may be obtained as compared with the case where a digital camera incorporating an area sensor is used. The resolution which can be obtained by the line sensor camera is such that a document appearing in a sheet of paper which is the size of DIN A4 can be legibly photographed.
In order to prevent an image from being distorted by irregular speed of displacement or revolution which occurs in an auxiliary scanning mechanism, it is known to detect the position of a moving part or the angular position of revolving part of the auxiliary scanning mechanism and adjust the timing of allowing a light from an object to be incident on each line. Irregular driving inevitably occurs to some extent, because the auxiliary scanning mechanism includes a mechanical drive mechanism such as means for rotating the mirror or means for moving a line sensor. Consequently, the image is distorted when the scan operation in order of successive lines is carried out with a fixed period, because the range which comes within the sweep of the line sensor varies with lines. The image expands when the object is scanned at a speed lower than the standard scanning speed, and contracts when the object is scanned at a speed higher than the standard scanning speed. This is the very reason why the timing of allowing the light from the object to be incident on each line is staggered in accordance with the irregularity of driving.
When all lines are scanned in consecutive order, longer time is required to photograph a frame. The longer the time required for photographing is, the more greatly the picture quality is affected by unintentional movement of the hands. Indeed the influence of unintentional movement of the hands will be mitigated by high-speed auxiliary scanning, but the high-speed auxiliary scanning will result in allotting only a short time for allowing the light from the object to be incident on each line such that, when the illuminance on the surface of the object is low, a decline in picture quality (i.e., signal-to-noise (S/N) ratio) will be caused by underexposure. Especially in case of indoor photographing, irregular color or photographic density will be caused by a flickering light source such as a fluorescent lamp.
There has been a prior proposal to cope with the aforesaid problem by the use of flashlight for a line sensor camera to make up for the shortage of illuminance. This prior proposal involves the emission of light having a truncated or flat-topped waveform. This light emission, which is hereinafter referred to as xe2x80x9cflat-topped light emissionxe2x80x9d, has been found advantageous when used for a line sensor camera so that the object may be illuminated over a period of about {fraction (1/10)} second, i.e., over a period of time required for scanning a frame. An excellent photograph in which all lines have equal brightness is obtained when the speed of switching action is high enough to keep pace with a period with which the lines are scanned. However, it is difficult to keep the switching action accurate. It should also be realized that it would be expensive to use a super high-speed switching element.
When an area scanning digital camera incorporating an area sensor is adapted to be kept in a serial imaging mode of operation as long as a release button is pushed, the object has to be illuminated with a certain quantity of light so that all frames may receive an equal quantity of light over a period of time during which the serial imaging is carried out.
The primary object of the invention is to provide an apparatus for obtaining a two-dimensional image of an object free of underexposure irrespective as to whether an object is scanned in order of successive lines or subjected to area scanning.
Another object of the invention is to provide an apparatus for obtaining a two-dimensional image of an object which allows a photograph uniform in brightness to be taken without recourse to the high-speed switching of illumination.
Still another object of the invention is to provide an apparatus for obtaining a two-dimensional image of an object which is less susceptible to the influence of a change in the quantity of ambient light.
Yet still another object of the invention is to provide an apparatus for obtaining a two-dimensional image of an object capable of minimizing or eliminating noise interference resulting from the switching of illumination and tending to cause the degradation of an image.
A further object of the invention is to provide an apparatus for obtaining a two-dimensional image of an object capable of preventing an image from being distorted by irregular scanning speed at the time of scanning the object in order of successive lines.
According to the aspect of the invention, there is provided an apparatus for obtaining a two-dimensional image of an object by means of an image sensor and a scanning mechanism, comprising a controller for commencing photographing after lighting a light source in response to a request for the commencement of photographing, and for allowing said light source to go out after the termination of photographing.